Electrophotography has been broadening its application in the field of copying machines, laser beam printers, etc. because of the advantages of high speed and high image quality.
A Carlson image formation system disclosed in U.S. Pat. No. 2,297,691 has generally been adopted in the current electrophotography using an electrophotographic photoreceptor. An electrophotographic photoreceptor used according to this technique repeatedly undergoes electrical and mechanical outer force through processes of charging, exposure, development, transfer, cleaning, destaticizing, and the like. In this situation, electrophotographic photoreceptors comprising inorganic photoconductive materials, such as selenium, selenium-tellurium alloys, and selenium-arsenic alloys, have conventionally been employed. On the other hand, photoreceptors comprising organic photoconductive materials have also been studied with attention being paid on their merits over inorganic photoreceptors, such as cheapness, productivity, and ease of disposal. In particular, organic photoreceptors of separate function type having a laminate structure composed of a charge generating layer which functions to generate charge on exposure to light and a charge transporting layer which functions to transport the generated charge are excellent in electrophotographic characteristics, such as sensitivity, charging properties, and stability of these properties on repeated use. Various proposals on this type of photoreceptors have been made to date, and some of them have been put to practical use.
While organic laminate type photoreceptors which have hitherto been developed are satisfactory in terms of the above-mentioned electrophotographic characteristics, there still remains an unsolved problem of durability against mechanical outer force in nature of the organic materials used. That is, being made of an organic material, a photosensitive layer tends to undergo wear or scratches on contact with a toner, a developer, a transfer medium (e.g., paper), a cleaning member, and the like and to suffer from adhesion of foreign substances such as toner filming, and the like, resulting in development of image defects. The working life of an organic photoreceptor has thus been limited.
Besides, with the recent advancement in color image formation and high-speed recording with copying machines and printers, the process involved has been getting more complicated, and the stress imposed on a photoreceptor has been increasing. From this viewpoint, too, the demand for improved sensitivity and durability of an electrophotographic photoreceptor has been increasing.
A number of measures have ever been proposed to improve durability of an electrophotographic photoreceptor. For example, various polycarbonate resins have been suggested as a binder resin for the surface layer of a photoreceptor (see JP-A-62-247374, JP-A-63-148263, JP-A-2-269942, JP-A-2-254459, and JP-A-3-63651; the term "JP-A" as used herein means an "unexamined published Japanese patent application").
Use of the known polycarbonate binder resins somewhat achieved improvement on durability, but the state-of-the-art photoreceptors are still unsatisfactory. That is, these resins have their several merits and demerits. For example, the most prevalent polycarbonate resin is soluble only in specific halogenated aliphatic hydrocarbons having a low boiling point. A coating composition prepared by using such a solvent causes whitening or orange peel of the coated surface. Further, since the solvent easily volatilizes before coating, the solid concentration of the coating composition increases to cause precipitation of the resin or additives, resulting in poor production yield. A coating composition if prepared by using other solvents undergoes gelation in a few days due to the low solubility, also resulting in poor production yield. Even in using polycarbonate resins having satisfactory solvent solubility, they have insufficient compatibility with a charge transporting material so that a coating composition containing a sufficient amount of a charge transporting material for obtaining desired electrophotographic characteristics tends to undergo precipitation of the charge transporting material.
Further, since many polycarbonate resins exhibit poor compatibility with a charge generating material, a charge generating material when dispersed with these polycarbonate resins is agglomerated, failing to provide a uniform coating film and to secure excellent image quality.